The photomultiplier tube is a kind of photo detector with excellent sensitivity and ultra-fast time response, which may be widely applied to equipments for photon counting, low-level -light detection, chemiluminescence, bioluminescence and the like. As a vacuum component, the conventional focusing type photomultiplier tube mainly comprises a photoemission cathode (also referred to photocathode), a focusing electrode, an electron multiplier and an electron collector (i.e. anode), wherein the photocathode is a very thin film made of a special photosensitive material deposited on a specific substrate, and may be classified into a transmission mode type and a reflection mode type according to the manner of photoelectric conversion.
Currently, the photocathodes of all the focusing type photomultiplier tubes are all transmission mode. The transmission mode photocathode is generally deposited on the inner surface of an input window at the top of the photomultiplier tube glass housing from which the light to be detected enters. As shown in FIG. 1, the operating process of this focusing type photomultiplier tube is as follows: when the incident photons pass through a front window of the transparent vacuum container 1 and impinge onto the photocathode 2, a portion of the photons are converted into photoelectrons, and the remaining photons penetrate the photocathode 2 and enter the vacuum container; a portion of the photoelectrons which have been converted from the photons in the photocathode 2 are absorbed by the photocathode 2, and the other portion of the photoelectrons (usually less than 30% of the total number of the incident photons) penetrate the photocathode 2, enter the vacuum container 1, are accelerated in the focusing electric field, and then enter a group of electron multipliers on the surfaces of which special materials are coated; the electrons which have been accelerated in the electric field impinge onto the surface of the electron multiplier electrode 3 to generate secondary electron emission. In this way, the multiplication of electron is achieved, and the multiplied secondary electron is collected by the anode 4 and then is output as a signal.
The above focusing type photomultiplier tube which adopts an electric field for focusing photoelectrons has a feature that the area of the photocathode is larger or much larger than that of the electron multiplier's surface for receiving photoelectrons, and such the feature is particularly suitable for fabricating a photomultiplier tube with a larger area. However, the conventional focusing type photomultiplier tube is often cylindrical or ellipsoidal; it is only possible to receive the light from the front by the transmission mode photocathode described above. In this case, the light is efficiently received within a space angle no more than 2π viewing angle, and the quantum efficiency for photoelectric conversion thus is low.
Furthermore, for the photomultiplier tube with a large area photocathode, the electron multiplier is generally the focusing dynode structure in FIG. 1, which is composed of a plurality of metal sheets provided with materials of high secondary electron emission coefficient on the surface. Such a focusing dynode structure is bulky, and is often located at the calabash-shaped rear opening at the lower part of the sealed vacuum container. For a large photomultiplier tube, this design results in some problems that there is substantial difference among the paths via which the photoelectrons emitted from the photocathode's surface arrive at the electron multiplier, and the distributions of the electric field which the photoelectrons experience are also different. As a result, the arrival time of the photoelectrons is also different, and it is difficult to obtain a satisfactory time response for a large photomultiplier tube.
It is necessary for the photomultiplier tube with a reflection mode photocathode to be provided with a substrate inside the transparent window of the vacuum container, and a reflection mode photocathode is deposited on the substrate. To cooperate with this reflection mode photocathode, it is required to apply an circular-cage type electron multiplier structure to implement the multiplication. Therefore, the effective area for receiving light of this photomultiplier tube is limited.
The photomultiplier tube also uses a microchannel plate as the electron multiplier. However, this kind of photomultiplier tubes using a microchannel plate are not focusing type, and the microchannel plate in the prior art is usually formed into the shape of a thin disk, so that it is impossible to achieve a relatively large area of the microchannel plate and it is required to place the microchannel plate adjacent to the photocathode. Since it is required that the area of the photocathode matches with that of the microchannel plate, the area of the photocathode is limited by the actually available microchannel plate.